The present invention relates to a clay mixture and a product made therefrom for use in confining or preventing the flow of seawater and other solutions containing cations, particularly cations found in industrial waste.
The importance of preventing the flow and leakage of toxic waste has become increasingly important. Proper containment of liquid industrial waste and sewage is critical to the continued health of our environment. As new processes are used in industry, new problems arise in treating the by-product of those processes.
Bentonite is a particularly well-suited clay for use in the formation of fluid barriers. When wetted, bentonite swells, or hydrates, by absorbing films of water that are thicker than those which form on other clays. The water absorbed by bentonite is retained even when subjected to high pressures. Bentonite is capable of swelling as much as ten to fifteen times its dry volume, and can absorb water to almost five times its own dry weight, while retaining its impermeability.
A very important characteristic of bentonite is that it will swell when uncontained, but will not exert significant pressure when confined against further swelling.
Various references acknowledge that swellable clay is generally suitable to form water impervious layers in a soil structure. Furthermore, a series of patents issued to Arthur G. Clem, i.e. U.S. Pat. Nos. 4,048,373; 4,103,499; 4,139,588; 4,467,015, suggest that a combination of additives is "absolutely necessary" for the proper performance of a bentonite layer in a seawater environment. The combination which is said to be essential includes a water-soluble dispersant, and a water-soluble polymer. An exemplary dispersant is said to be a salt of phosphoric acid. Alternative dispersants suggested in the Clem patents include salts of alkaline earth metals. An example of the other essential element mentioned in the Clem patents is polyacrylic acid. There is no clear explanation of the theory behind the selection of these "essential" components of the mixture.
It is believed that the contaminating effects of seawater and other solutions, such as sewage leachate and paper pulp sludge, upon the swellability of a bentonite layer is due to the presence of positively charged ions, such as calcium ions, which have double charges. It is understood that bentonite forms a multi-layer crystalline structure in the presence of water, and in the presence of pure water its ability to swell is at a maximum. In sodium bentonite, sodium molecules are subject to attack or replacement (through ion exchange) by cations having a double-charge. The resulting removal of sodium from the sodium bentonite structure is believed to cause a reduction in the ability of the bentonite layer to perform as an impermeable layer. Thus, in the presence of seawater or other solutions which may contain various concentrations of cationic contaminants, the ability of the bentonite to perform effectively is inhibited.
It has been found that by significantly increasing the ratio of single-charged ions, such as sodium ions, to the number of detrimental cations, such as calcium ions, the ability of bentonite to swell in the presence of seawater can be improved. It is believed that by substantially outnumbering the deleterious double-charged cations with single-charged sodium cations, the chances for replacement of double-charged for single-charged cations is reduced. This may result from mere distribution or may be enhanced by the repulsion of particles having like charges. By surrounding the bentonite structure with "friendly" sodium ions, the integrity of the structure is protected. It is believed that the same or similar detrimental double-charged cations, which give seawater its contaminative potential, are present in solutions such as paper pulp, sludge, and industrial waste of various kinds, particularly those containing double-charged cations of zinc, cadmium, nickel and other metals.
Contrary to the teachings of Clem, as discussed above, it has been found that the presence of a water-soluble polymer is not essential to the formation of a mixture which will resist the detrimental effects of seawater.
U.S. Pat. No. 4,501,788 (the '788 patent), which is assigned to the assignee of the present invention, shows a particularly useful form in which the mixture of the present invention can be used. The '788 patent describes a flexible sheet comprising a support coated with a swellable bentonite. The support is a porous flexible layer of fabric. Such sheets are used to line ponds and lagoons and landfills, tank farms and hazardous waste sites. By using a sheet of swellable bentonite, an impervious layer is formed to prevent hazardous leachate from entering adjacent groundwater systems.
An important object of this invention is to provide an inexpensive and effective barrier to the flow of solutions through a soil structure or foundation.
Accordingly, a further object is to provide a mixture comprised of the minimum number of components necessary to resist the degradation of the clay layer by seawater and other solutions containing cations.
Still another object of the present invention is to enhance the contamination resistance of sodium bentonite clay with the addition of single-charged cations, which will tend to neutralize the contaminative effects of seawater and other cation carrying solutions.
These and other objects of the present invention will be apparent from the following detailed description of an embodiment of the invention.